1. Field of the Invention
The present invention relates to the area of apex-locating methods and devices which are used in endodontics to locate, in a root canal of a tooth, the position of the apex in terms of depth, i.e. the position of the summit of this root (i.e. apical terminus) and, more precisely, the end of the orifice of the apical foramen at the bottom of the root canal.
2. Description of the Related Art
During dental surgery procedures, in particular during a procedure to clean and shape the root canal, the apex locators serve to avoid crossing the apical foramen, i.e. passing the apical terminus and to keep the subjacent maxillary ligament with its nerve bundles from being reached.
FIG. 1 shows the anatomical structure of a tooth in a schematic view along a plane of cut along the axis of the canal of a root of the tooth. Certain teeth, such as the molars and premolars, can have a number of roots RT or at least several root canals CR which may be separate or joined.
The end of the root RT is pierced by an orifice FA known as the apical foramen for passage of nerve bundles and vessels. Sometimes, as shown in FIG. 1, this orifice FA at the end of the root canal CR narrows to form a bottleneck at the apical constriction CA (narrow neck permitting passage of the group of vessels and nerves which irrigate the pulp). In other cases (not shown) the root canal has a large cross-section with no narrowing.
At this apical constriction CA, there is located the cement-dentine joining interface CT/IV, an interface between mineral substances (cement/dentine) which have contrasting electrical properties.
During endodontic surgical procedures, such as procedures for cleaning and shaping the dental canal CR, dentists seek to remove all materials, debris and organic fluids which fill the root canal CR right to the bottom, i.e. as far as the end of the apical foramen FA, in order to avoid a dental abscess recurring in the root canal CR.
However, the dentist's objective is principally, as far as possible, not to pass the apical terminus APX, on the one hand, so as not to cause the patient any pain and, on the other hand, so as not to hollow out a cavity below the root, beyond the apex, which could give rise to the development of an abscess.
It is thus of the greatest importance for the dentist to locate the foramen FA and apical terminus APX very precisely.
As indicated in FIG. 1, radiographic images of the teeth taken along the horizontal plane XRA of the jaw generally give an incorrect radiographic position for the apex which does not correspond to the true position of the directing plane of the anatomical apex AA.
Electronic apex-locating devices have been developed for the past fifty years to locate the end of the root canal in a precise manner, being based on the changes in electrical properties in this transition zone.
The first generations of apex locators, developed by Sunada on the basis of the work of Pr. Suzuki, operate on a principle of resistance measurement in the root canal, being based on the observation that when the apical zone is crossed, the resistance value drops suddenly and crosses a resistance threshold of about R=6.5 kΩ, a value which is substantially constant from one individual to another.
As shown in FIG. 2A, the resistance is measured between a first electrode ES formed by an endodontic file or probe inserted into the root canal CR and a second electrode EM shaped to be brought into close electrically conductive contact with an oral mucous membrane (lip, gum . . . ).
FIG. 2B indicates that the resistance R first drops slightly as the endodontic file ES is pressed in to a depth DP within the root canal CR on the axis of the tooth, then R drops sharply when the apical zone is crossed, before returning to a base value once the apical terminus has been passed.
Sunada established that the apex is located in the zone where the resistance crosses the threshold value R=6.5 kΩ, a value which is substantially constant from one individual to another.
U.S. Pat. No. 5,096,419 in the name of Kobayashi of the company MORITA cites two prior art Japanese documents JP 2817/62 and JP 25381/62 relating to two series of measuring devices making it possible to locate the position of the apex and to determine the depth of the root canal.
The first series of devices is based on a resistance measuring principle using direct current, the continuous resistance dropping sharply when the apical zone is crossed.
The second series of devices is based on an impedance measuring principle, resistance generalisation, but with measurement using an alternating signal and including two resistive and capacitive components; the alternating signal impedance drops when the point of the probe approaches the apex.
The first resistance measuring principle only makes it possible to detect when the apical terminus is passed, which does not satisfy the dentist's objective of being warned before having passed the apex.
The second impedance measuring principle should prove to give more warning because the impedance is supposed to drop when there is a change in properties at the cement/dentine interface CT/IV when the apical constriction CA is being crossed, this being at a location before the directing line AA of the apex is reached as shown in FIG. 1.
A first disadvantage is that this second measuring principle, based on the detection of a drop in impedance at the cement/dentine junction of the apical constriction does not work on children and young patients because their teeth have little or no hypermineralised dentine.
In general, these two series of devices necessitate delicate rating and calibration operations, operations which are imprecise, tedious and a source of error.
In practice, and speaking generally, these two series of apex locators have the disadvantage of indicating the position of the apex only after the point of the electrode probe has crossed the apical constriction. The resistance measurements do not drop before the point of the probe has passed the apical terminus APX. In fact it proves to be the case that the impedance measurements drop only when the point of the file has passed the orifice of the apical foramen FA and touches the ligament below the dental root RT. However, dentists seek most particularly not to cross the apical foramen FA.
Another considerable problem is that the two measuring principles of these two series of apex locators have the disadvantage that the resistance/conductance measurements become wholly imprecise, even nonsensical, in the presence of conductive fluids in the root canal.
During dental cleaning and shaping procedures the canal is generally filled with fluids and materials, in particular organic bodies and matter (saliva, blood, lymph, serum, physiological fluids, organic debris) which behave like slightly salty media which are thus fairly conductive, analogous to what is known as physiological liquid or serum (common aqueous saline solution of 0.9% NaCl) which is a moderately conductive ionic solution like seawater.
Furthermore, dentists have to continually clean the mouth of the patient with a flow of rinsing liquid based on a conductive saline solution of NaCl, and especially with disinfectant solutions, in particular Dakin's liquid (“neutral diluted solute of sodium hypochlorite”, NaClO diluted to 2.5% or 5%, similar to true Javel water) which is a very highly conductive ionic solution (OH-ions). Such highly conductive ionic solutions totally disrupt conductivity measurements (resistance, impedance) and entirely invalidate any determination of the position of the apex.
The on-going presence of such organic fluids and solutions during dental procedures precludes the use of apex-locating devices based on such resistance or conductance measuring principles.
The improvement of this prior art proposed by Kobayashi in document U.S. Pat. No. 5,096,419 involves comparing two conductance measurements effected at two distinct frequencies f and 5f as shown in FIG. 3 in order to be unencumbered by fluctuations in conductivity caused by the presence of ionic solutions.
According to this third measuring principle, measurements of voltage (V) are carried out at the terminals of a reference resistor R=5 kΩ placed in series with the electrodes. The series circuit is supplied by a generator of square signals at the frequency f, which produces harmonic signal components at the frequencies fa=f and fb=5f. In a first time period (phase I) during the insertion of the probe into the root canal, the voltage measurements A and B taken at the two frequencies fa=f and fb=5f remain stable. In a second time period (phase II), the two voltage measurements A′ and B′ increase as a zone II corresponding to the apical constriction is being crossed (because the impedance of the canal drops as the cement/dentine junction is approached).
According to document U.S. Pat. No. 5,096,419 of Kobayashi, the two curves A′ and B′ are not equidistant in zone II but their deviation F decreases.
According to Kobayashi the difference δ between the two voltage measurements A and B (δ=A−B), initially of the substantially constant value −Γ in zone I, would decrease in zone II.
Kobayashi states that in zone II, the deviation B−A or the difference δ=A−B comes closer to an extremum value (minimum deviation) before shifting suddenly in the other direction and becoming more separate again. The extremum, i.e. the point AX where the deviation |B−A| is minimum (i.e. δ=A−B max.) corresponds to the position of the apex according to the teaching of U.S. Pat. No. 5,096,419.
Document U.S. Pat. No. 5,096,419 then describes a sophisticated electronic circuit for threshold detection in order to determine at which point the difference δ=A−B between the two voltages measured at the two frequencies fa=f=1 kHz and fb=5f=5 kHz crosses a threshold value θ corresponding to the position of the apex.
The disadvantage of this device is that the fixing of the threshold value θ still necessitates calibration operations which are delicate, imprecise and a source of error. In practice, the precise course of the variations in the curves A and B and their deviation |δ|=|A−B| are eminently variable according to the individual concerned and the electrical conditions prevailing in each root canal.
For each individual canal of each root of each tooth it is necessary to recommence the rating and calibration operations, operations which are specialised, time-consuming and tedious for the dentist and which make these devices unattractive for the dentist to use.
In fact, depending on whether the threshold is fixed at a value θ below the extremum or at a value θ′ beyond the extremum, either the measurement of the position P of the apex AX is imprecise and encumbered with an error ε, or no crossing of the threshold is detected and the device does not signal that the probe is passing the apical terminus.
Generally speaking, this third principle of detecting a difference in measurements made at two frequencies also has the disadvantage of not setting an absolute measuring criterion for the position of the apex.
The detection of the crossing of a threshold still has the disadvantage of being relative to the setting of an arbitrary threshold value.
From another point of view, if it were desired to detect the point AX of turning back, i.e. the point of inflexion AX where the curve δ=A−B reaches the extremum and changes the direction of variation, which would constitute an absolute criterion, it would nevertheless be necessary to pass the point AX, i.e. to cross the apical terminus in order to detect the passage at the extremum and the change in the direction of variation.
FIG. 4 illustrates a fourth apex-locating principle proposed by document U.S. Pat. No. 5,080,586 in the name of Kawai of the OSADA Institute.
Document U.S. Pat. No. 5,080,586 describes a measuring system comparable to that of document U.S. Pat. No. 5,096,419 and consisting of applying two alternating voltages V1 and V2 having two distinct frequencies f1 and f2 to the terminals of a circuit comprising two electrodes (a needle inserted into the root canal of a tooth and an electrode in contact with an oral mucous membrane) in series with a measuring resistor.
It is the case that the two frequencies f1=1 kHz and f2=5 kHz proposed by document U.S. Pat. No. 5,080,586 are identical to the two frequencies fa=f=1 kHz and fb=5f=5 kHz used according to the teaching of the other document U.S. Pat. No. 5,096,419.
On the other hand, according to FIG. 4 which shows the course of the measurement curves of document U.S. Pat. No. 5,080,586 of the prior art, the measurement curves of the two voltages V1 and V2 taken at the two frequencies f1=1 kHz and f2=5 kHz diverge and move apart continuously with the depth P of insertion of the electrode, the deviation (V2−V1) increasing monotonously.
The points of view on the course of the voltage curves plotted at the two frequencies of f=1 kHz and 5f=5 kHz are thus divergent and show the degree to which the measurements are errant, unreliable and do not constitute an absolute measurement criterion for precisely determining the position of the apex.
In order to determine the position of the apex, the document U.S. Pat. No. 5,080,586 proposes determining the ratio between these two voltages V1 and V2 plotted at the two frequencies f1 and f2 (ratio V2/V1) and determining a threshold value, the position of the apex A corresponding to the crossing of this threshold by the ratio V2/V1.
This alternative measuring principle still has the disadvantage of not constituting an absolute criterion for determining the exact position of the apex but of referring to relative threshold values, varying according to the individuals concerned and the electrolytic conditions prevailing in each root canal, which means that the dentist has to perform calibration operations which are delicate, imprecise and a source of error.
More generally, these latter apex-locator generations are based on principles of measuring voltage at the terminals of a reference resistor in series with the two electrodes which reflect the conductance (inverse of the impedance) existing in the root canal between the electrodes.
The problem is that such measuring principles are directly affected by the presence of conductive fluids in the root canal which entirely invalidate the determination of the position of the apex.
As already mentioned, the presence of conductive fluids in the root canal is inevitable in dental surgery procedures because of the presence of fluids and organic materials (blood, lymph, saliva, serum, organic debris) and the necessity of cleaning the mouth with rinsing solutions (physiological liquid, i.e. 0.9% NaCl solution) or with disinfectant solutions (Dakin's liquid, i.e. NaClO solution).
Moreover, another general problem of the apex-locating devices based on measurements of impedance in the root canal of the tooth is that they do not permit resolution of complex teeth having several root canals or root canals with bifurcations or aberrations (multiple, forked, branched or twin roots, excrescences . . . ).
The molars have several roots and root canals which are generally well separated. The premolars and molars generally have twin roots, just subdivided at their end by a bifurcation into two twin (forked) root canals. Other teeth may have branches or aberrations. The teeth which generally are most subjected to dental surgery and endodontic cleaning and shaping procedures are precisely these complex teeth, notably the molars and premolars.
The object of the invention is thus to provide a means of apex location which solves these problems and overcomes the disadvantages of prior art apex locators.